WASHINGTON (CNN) -- Former New York City Mayor Rudy Giuliani is the clear front-runner in the crowded GOP presidential field and has a substantial lead on the closest competition, according to a CNN poll released Monday.

Giuliani is the preferred candidate for 34 percent of likely Republican voters, according to the poll, conducted by Opinion Research Corp.

Giuliani has a double-digit lead over his nearest rival, Sen. John McCain of Arizona, who is the choice candidate for 18 percent of potential Republican voters, according to the poll, which had a sampling error of plus or minus 5 percent. (Key results)

Giuliani has made noticeable gains in the GOP race since late January, when he polled at 32 percent and McCain at 26 percent. (Polling details)

McCain's support has slumped in the past few weeks and now sits at its lowest point in six months. His campaign, however, said the senator is happy with his role as underdog because it will allow him to surprise the front-runner, as he did during his campaign against President Bush in 2000, when he won the New Hampshire primary.

Conservative endorsements

In a move to attract conservatives skeptical of his position on social issues such as gun control, abortion rights and same-sex civil unions, Giuliani's campaign Monday announced endorsements from Sen. David Vitter, R-Louisiana, and former Solicitor General Ted Olson, whose wife, Barbara, was killed in the 9/11 terrorist attacks.

"Obviously, I disagree with Rudy on some significant social issues, and these are very important to me and to many people I represent," said Vitter, a vocal opponent of abortion, gay marriage and gun control.

"But after numerous personal meetings with the mayor, it's very clear to me that he's not running for president to advance any liberal social agenda," he said.

Olson, who served with Giuliani in the Reagan Justice Department, said he believes the former mayor "will appoint the kind of judges that we expect in this country -- people that respect the rule of law and the role of judges and who interpret the laws and interpret the Constitution, instead of judges who want to make policy." (Watch Giuliani discuss judicial appointments)

Monday's endorsements are a part of Giuliani's strategy to focus on issues where his stance is attractive to social conservatives -- such as fighting terrorism, offering strong leadership, restraining federal spending and appointing "strict constructionists" to the federal bench -- rather than social issues where they disagree.

"There are things that unite us that seem to be a lot stronger," Giuliani said.

Former Massachusetts Gov. Mitt Romney and former House Speaker Newt Gingrich tied for third in the poll, with 9 percent support among likely Republican voters. None of the other candidates broke 3 percent.

While the poll found Giuliani's margin widening, there were also indications that the increased scrutiny on the former mayor has caused his popularity to slip.

In November, 60 percent of likely American voters had a favorable view of Giuliani and only 18 percent viewed him unfavorably. His favorable rating has since dropped 4 percent and his unfavorable rating has risen to 21 percent.

Gingrich, who has said he will likely decide by September whether to seek the GOP nomination, was the least popular figure among the leading candidates, with 25 percent viewing him favorably and 43 percent unfavorably.